The retroreflective sheeting is used for signboards such as road signboards and construction site signboards, car license plates of automotive vehicles such as automobiles and motorcycles, and safety materials such as collision warning signboards, clothing, and life jackets.
There are proposed some techniques of obtaining retroreflection by embedding micro glass beads in a resin sheet and utilizing refraction of the glass beads (see Japanese Unexamined Patent Publication No. 6-160615, Japanese Unexamined Patent Publication No. 6-347623, and Japanese Unexamined Patent Publication No. 9-212115). In the proposed techniques, even if an angle (hereinafter, called as “incident angle”) formed between a vertical line perpendicularly intersecting a surface of the resin sheet, and light incident on the resin sheet is increased, lowering of retroreflective performance (sometimes called as “coefficient of retroreflection”) is suppressed (in other words, superior incident angularities are obtained). However, in the above arrangement, the absolute value of a luminance factor (i.e. a coefficient of retroreflection) is small, which obstructs acquiring sufficient retroreflection. In addition to the above drawback, since segregation of the glass beads from the resin is impossible, segregation recycling is impossible, and incineration is impossible. Accordingly, disposal by landfill is the only measure, which may increase environmental load.
In order to solve the above drawbacks, a triangular-pyramidal cube-corner retroreflective sheeting with elements has been proposed. With the retroreflective sheeting, segregation recycling is possible, which may reduce environmental load. Also, the retroreflective sheeting provides an improved luminance factor (i.e. an improved coefficient of retroreflection), and accordingly, provides superior retroreflection against incident light of a specific incident angle. However, the incident angularities are poor. In other words, the retroreflective sheeting above exhibits a desirable retrorefliective performance while the incident angle is small, but the retroreflective performance is sharply degraded as the incident angle is increased.
In the triangular-pyramidal cube-corner retroreflective sheeting above, reflected light is less likely to be diffusely reflected in a wide angle, as compared with the glass beads type retroreflective sheeting. Accordingly, in practical use of the retroreflective sheeting above, in the case where light emitted from a headlamp of an automobile is retroreflected on a traffic signboard, for instance, the retroreflected light may be hard to reach the driver's eyes if the driver is located on an off-axis position of the optical axis of the retroreflected light due to the narrow diffuse angle of the retroreflected light. The drawback becomes conspicuous particularly when the automobile comes close to the traffic signboard, because the angle (hereinafter, called as “observation angle”) formed between the axis of the incident light and the axis (observation axis) connecting the driver's position and a reflected point of the incident light on the traffic signboard is increased (that is, observation angularities are degraded).
A retroreflective sheeting obtained by arranging triangular pyramid type reflective elements of various shapes on a thin sheet, and a method for producing the retroreflective sheeting are proposed as an improved approach to solve the above drawbacks (see U.S. Pat. No. 2,481,757). It is described that examples of the triangular pyramid type reflective elements include a triangular pyramid type reflective element, in which the apex of a triangular pyramid is aligned with the center of a base triangle, without a tilt of the optical axis, and a triangular pyramid type reflective element, in which the apex of a triangular pyramid is off the center of a base triangle, with a tilt of the optical axis, to efficiently reflect light against an approaching automobile. The method in the publication above, however, has no specific disclosure about a microsized triangular pyramid type reflective element, and has no recitation on a desirable size and a desirable optical axis tilt of the triangular pyramid type reflective element.
Some approaches are proposed as a measure to solve the above drawbacks by specifying the size of the triangular pyramid type reflective element, and determining a tilt of the optical axis (see Japanese Unexamined Patent Publication No. 6-250006 and Japanese Unexamined Patent Publication No. 2001-264525). However, the improvement is insufficient, and in the case where the coefficient of retroreflection when both the incident angle and the observation angle are small is exceedingly large, halation which makes the recognition of a sign or the like difficult for a driver may likely occur due to too bright reflected light; whereby the driver may be misguided. On the other hand, in the case of where the retroreflection is to be improved when the incident angle is larger, the retroreflective performance (i.e. the coefficient of retroreflection) may be degraded when both the incident angle and the observation angle are small, which may fail to provide sufficiently enhanced incident angularities.
Also, an attention should be paid in attaching a retroreflective sheeting on a base member in view of a drawback that the retroreflective performance is extremely varied between vertical direction and horizontal direction, in other words, direction characteristics are poor. So far, there is no specific technical disclosure to solve the drawback.
Furthermore, it is necessary to provide protruding supports as a constituent member of the retroreflective sheeting having a triangular pyramid type reflective element layer in order to secure an air layer on the back side of the triangular pyramid type reflective elements in firm contact with a backing film (or a backing sheet). However, the reflective element layer having the protruding supports in the firm contact with the backing sheet cannot meet the total internal reflection requirement. Therefore, the retroreflective performance of the retroreflective sheeting with the triangular pyramid type reflective element layer may be degraded.
Moreover, since according to the retroreflective sheetings produced by the aforementioned methods, the triangular pyramidal element has a very sharp apex, in attaching the backing sheet, the sharp apex may be abraded or deformed, which cause undesirable fluctuation of the retroreflective performance, and further, when the retroreflective sheeting is temporarily rolled in after the prism elements are formed, the triangular pyramidal elements may be abraded in the roll, and consequently broken. Since the portion containing the broken prism elements is unusable as a product, and should be disposed of, environmental load may be increased.